The send volume levels actually only affect trim/read setting, but do not affect the automated level of the send volume. Whatever automation mode I use, only that particular non-automatable level is changed. The envelope is not affected.

This meas I can't ride the send volume with the faders. Maybe it's a different command, or Justin changed something ?

I'm using Reaper 5.17 and 5.18pre3 x64 on Win7x64 using a single MCU midi-based unit.